Electrical wave filter



May 21, 1940.

B. TREVLR ELECTRICAL WAVE FILTER Original Filed Jan. 4, 1937 2 Sheets-Sheet 1- INVENTOR BERTRAM TREVOR K Z ATTORNEY May 21, 1940.

B. TREVOR I ELECTRICAL WAVE FILTER 1 Original Filed Jan. 4, 1937 2 Sheets-Sheet 2 MWER INPUT 7/ ITEM/M11370 TRANS/W775 INPUT FROM PUWER LINE 477015 FREQUENCY MC "QINVENTOR a??? TREVOR m ATTORNEY Patented May 21, 1940 UNITED STATES 2,201,326 ELECTRICAL WAVE FILTER I Bertram Trevor, Riverhead, N. Y., assignor to Radio Corporation of America, a corporation of Delaware Application 22 Claims.

This invention relates to electrical wave filters, and particularly to low pass wave filters.

It is customary, in the radio field, to employ a single source of power supply for energizing one or more transmitting circuits. Where a receiver is located near a transmitter and fed from the same source of power, precautions are often necessary to prevent radio frequency energy, generated by the transmitter, from being fed back along the power line to the receiver. If such preventive measures are not taken, any radio frequency energy present on the power line may prove very disturbing to the receiver operation. Heretofore it has been customary to use a filter of the coiled inductance type for this purpose. Such a filter, it has been found, cannot be very readily designed to be heavy enough to carry hundreds of amperes of current while at the same time providing suitable attenuation for the ultra high frequencies, due mainly to the high distributed capacitance of the coiled inductance.

The present invention provides a more effective method of preventing the aforementioned disturbances and has for one of its objects to provide a low pass electrical wave filter which will give high attenuation at extremely high frequencies, preferably a sharp rise of attenuation immediately above the nominal cut-off frequency and low attenuation below the nominal cut-off frequency. Another object is to provide a wave filter which will give a desired attenuation at ultra high frequencies, while at the same time enabling the passage of high currents through the filter at ordinary power frequencies.

The design of low pass filters is well known and has been set forth by Shea in his book entitled Transmission Networks and Wave Filters, published in 1929 by Van Nostrand Company, Inc., New York. Page 291 of this book shows the formulas involved in concise form. In the design of low pass filters it is well known that it is desirable to provide reactive elements having no loss, as under this condition there will be no attenuation up to the nominal cut off frequency r usually designated by fc, assuming the filter is terminated with its image impedance later described. Actually, it is never possible to provide lossless elements. This condition gives rise to attenuation of frequencies below fo, the amount depending upon the power factorof the filter re:

January 4, 1937, Serial No. 118,953 Renewed July 26, 1939 actances. In the present invention it is entirely feasible to use reactive elements having a power factor of from .001 to .0001 (i. e., a Q of 1000 to 10,000). These figures represent extremely low losses and this low loss is a decided advantage obtained with the type of construction hereinafter described, in that little attenuation is experienced below the frequency fc and a sharp rise in attenuation with frequency is produced immediately above the frequency fc.

In general, a preferred form of filter of the present invention comprises a concentric line affair having an inner conductor and an outer conductor directly connected together at one end. The inner conductor is designed to be the series element in the filter. The filter is so designed that the constants thereof conform with well known practices as set forth, for example, by Shea in his book Transmission Networks and Wave Filters, supra. The outer conductor is capacitively coupled throughout its length to a surface of zero radio frequency potential, such as ground. In this way, the outer conductor gives a desired degree of shielding for the inner conductor while also improving the power factor of said inner conductor (i. e., a lower power factor), as compared to a series element without a surrounding outer. conductor, for a condition in which the outer conductor is large in diameter with respect to the inner conductor.

Although the electrical wave filter of the present invention is hereinafter described with particular reference to its use between the power line and the input terminals of the high frequency apparatus, it is to be distinctly understood that it is not limited to such use since the filter is equally applicable for use between any two stages, either in a transmitter or in a receiver circuit for passing certain low frequencies and rejecting one or more other high frequencies.

A more detailed description of the invention follows in conjunction with the drawings, where- Fig. 1 illustrates diagrammatically and in cross section a preferred form of filter using a midshunt section;

Fig. 1a shows an equivalent electrical circuit of a filter of Fig. 1;

Fig. 2 illustrates diagrammatically and in cross section a second preferred form of filter using a mid-shunt section;

Fig. 2a shows the equivalent of the filter of Fig. 2; V

Fig. 3 illustrates diagrammatically and in cross section a type of filter similar to that of Fig. 1 using a mid-series section; i

Fig. 3a shows the equivalent electrical circuit of the filter of Fig. 3;

Fig. 4: illustrates diagrammatically and in cross section a preferred form of low pass filter using a mid-series section;

Fig. 4a shows the equivalent electrical circuit of the filter of Fig. 4;

Fig. 5 illustrates diagrammatically and in cross section another form of low pass filter using a mid-shunt section;

Fig. 5a shows the equivalent electrical circuit of Fig. 5;

Fig. 6 illustrates a practical embodiment of the filter of Fig. 1 in connection with a transmitting circuit; and

Fig 7 is a graph showing the frequency versus attenuation characteristic of a particular filter constructed in accordance with the invention, which has given satisfactory results.

Referring to Fig. 1, there is shown an elec-, trical wave filter of the concentric line type comelectrical circuit prising an inner conductor Iand an outer conductor 2, connected together at one end by an end plate 3. The inner conductor which is the series element in the filter has an inductance which is determined both by its length and the ratio of diameters of outer and inner conductors. The value of this inductance designated by L may be approximately calculated from the well known formula:

L=Z4.605 X 10 log g microhenrys where b is the inside diameter of the outer conductor 2, a is the outside diameter of the inner conductor 5, and l is its length in centimeters. This formula does not take into account the bend .at the end of conductor I as this factor must be considered otherwise. 2 is shown capacitively coupled to a surface of zero radio frequency potential 5, such as ground, by means of one or more capacitors l. The end of inner conductor I which is remote from the end directly connected to the outer conductor, is also capacitively coupled to surface 5 through a capacitor 6. The terminals of the filter are designated I and 8, respectively, which terminals can be interchangeably used either as input or output terminals.

Where the filter of the invention is to be used to carry heavy currents, of the order of many hundreds of amperes, the inner conductor, which is used as a series element, and the end plate through which the currents also flow, are designed tohave large cross-sectional areas to prevent undue losses in the filter. The outer conductor, however, need not have a large thickness, but may merely comprise, if desired, a thin metallic sheet surrounding the inner conductor. This sheet, of course, may be either cylindrical or square, or any surface approaching these configurations.

Fig. 1a shows the electrical equivalent circuit of Fig, 1, in which the inductance of the inner conductor I corresponds with L, the capacitances 4 taken together correspond with be adjusted to a desired value.

Outer conductor and the capacitance 6 corresponds with the other capacitance ml Q R 1 =17, =iTrc where R is the impedance that will terminate,

the filter section without reflection as the frequency j approaches zero, fc is the nominal cutoff frequency above which the filter gives attenuation, and Z: is the terminating impedance which will give no reflection at any frequency J within the pass band. These relations may then be applied to the design of the filter of Fig. l. I 7

Referring to Fig. 2, there is shown an electrical wave filter of the concentric line type comprising an inner conductor I terminating at one end in an enlarged portion 9, and connected to the outer conductor 2 at its other end by the end plate 3. The enlarged portion 9 is designed to provide capacitive coupling between the end of inner conductor I and the outer conductor 2. The movable plate I0 allows this capacitance to The outer conductor '2 is capacitively coupled to a surface of zero radio frequency potential 5, such as ground, by means of one or more capacitors 4. The en'- larged portion 9 of conductor I is capacitively coupled to the surface of zero radio frequency potential 5 by means of capacitor 6. The terminals of the filter are designated I and 8, which terminals can be interchangeably used either as input or output terminals.

A better understanding of the filter of Fig. 2 may be had by referring to Fig. 2a which shows the electrical equivalent circuit to which Fig. 2 corresponds. Here (Fig. 2a), the inductance L1 with the associated capacitance C1 corresponds with the inductance of inner conductor I with its associated capacitance between the enlarged portion 9 and the outer conductor of Fig. 2. As in the case of Fig. 1, the inductance of the inner conductor I may be calculated from the formula mentioned above,

where b is the inside diameter of the outer conductor 2, a is the outside diameter of the inner conductor I, and Z is its length in centimeters not including the length of the enlarged portion 9. It is here assumed that the diameter of the portion 9 is nearly equal to the inside diameter of the outer conductor 2 as this assumption allows theinductance of the portion 9 to be neglected, inasmuch as it will be small compared with that of conductor I. It is to be understood that the diameter of 9 may have any value and its inductance taken into account in the calculations. InFig. 2a. we see the two shunt capacitances 02/2, which correspond with the capaci- .tances 4, taken together, and capacitance 6, respectively, in Fig. 2;

Inspection of Fig. 2a shows it to be the well known shunt-derived m-type low pass filter using the mid-shunt section, as set forth by Shea on page 291 of his book mentioned above. The formulas given are herewith shown:

M/ 'ITC'T 401) where L and C are taken from the constant 70 type of filter section also shown on page 291, in which:

on The relationship shows that the frequency foo is reached when the two elements L1, C1 are in resonance, and at this frequency the filter gives maximum attenuation.

The m-derived type of filter has the advan tage of enabling the designer to set the frequency of maximum attenuation, f o, at the most desirable frequency depending upon his particular requirements. The filter arrangement of Fig. 2 is then a novel method of constructing a midshunt section of a shunt-derived m-type low pass filter primarily for ultra high frequencies possessing all of the flexibility of the original type as set forth by Shea. The construction shown in Fig. 2, as well as that of Fig. 1, allows the use of reactive elements having extremely low loss as mentioned earlier, because concentric line sections may be easily built to have a power factor of .0001 (Q=10,000) in the ultra high frequency range. It is also possible to construct heavy capacitors having similar values of power factor. The enlarged portion of inner conductor i, Fig. 2, is an example of this. The advantages of such low loss elements were described above.

It may be seen that the filter shown in Fig. 1,

of the constant k type, could be considered a limiting case of the m-derived type, as the inner conquencies in the neighborhood of fc and below. The constant 70 type of low pass filter may be considered to be the m-derived type with the value of m equal to unity.

In this connection, it should be mentioned that as the frequency is increased above the frequency corresponding to that for which the inner conductor of Fig. 1 is electrically a quarter of a wavelength long, this conductor is in effect a capacitive reactance between its terminals. Attenuation still exists in this region. As the frequency is still further increased, the inner conductor of Fig. 1 eventually becomes one-half a wavelength long. Under this condition the conductor becomes an element of nearly zero'impedance. At still higher frequencies, the inner conductor again becames inductive and a narrow theoretical band pass occurs, assuming proper values of terminating impedance. However, the image impedance of the filter within this band pass has dropped to such a low value that ordinary terminating impedances would give rise to extremely high reflection losses so that in all probability considerable attenuation would actually exist. At higher frequencies, the attenuation rises and reaches another maximum at the frequency corresponding to that for which the inner conductor becomes three-quarters of a wavelength long. This rise and decrease of attenuation from maximum to minimum will repeat itself at odd and even multiples of onequarter wavelength, respectively.

Referring to Fig. 3, we have a low pass filter of the constant is type employing the mid-series section as contrasted with the mid-shunt section of Fig. 1. The two types of section give identical attenuation characteristics and the element values are determined by the same formulas, but the mid-series image impedance is given by the following relation within the pass band:

It will be noted in this figure that the capacitance between the outer conductor 2 and the surface of relatively fixed or zero radio frequency potential has been uniformly distributed over the length of the outer conductor by mounting same on a suitable metallic plate P which is spaced away from or insulatingly positioned with respect to another metallic plate P in contact with the surface of fixed radio frequency potential.

Fig. 3a shows the equivalent electrical circuit of Fig. 3.

In Fig. 4 is shown a low pass filter of the shuntderived m-type employing the mid-series section as contrasted with the mid-shunt section of Fig. 2. These two types of section also give identical attenuation characteristics and the element values are determined by the same formulas, but here the mid-series image impedance is given by a different relation within the pass band; namely:

It is important to note that the image impedance Z: given by this relation remains at nearly a constant value throughout the band pass if the values of is and fog are chosen to give a value m in the neighborhood of .6. It is desirable to keep the image impedance as nearly constant as possible with frequency, in order that the filter terminal impedances, which are normally relatively series section of the shunt-derived m-type of low pass filter in the filter terminations in order to take advantage of this desirable feature.

In Fig. 411 we see, the electrical equivalent circuit of Fig. 4.

In both Figs. 3 and 4 the shunt capacitance C and C2 respectively may, if desired, be composed of one or more capacitances in parallel, as is shown in Figs. 1 and 2.

Fig.5 shows a suggested arrangement for using concentric line sections to construct a seriesderived m-type low pass filter using a mid-shunt section. Fig. 5a shows the electrical equivalent circuit. The appropriate formulas may be found on page 291 of Shea, supra.

Under conditions where it is desired to employ the filter of say Fig. 1 merely to reject the ultra high frequencies and pass only the lower power frequencies, the filter need not be terminated in its image impedance by the associated circuits in the pass band, since there would be no appreciable attenuation of the power currents under mismatched conditions. Under these conditions, it would usually be desirable to construct the filter to have an image impedance much lower than the impedance of the associated terminal circuits as this would increase the attenuation in the region beyond the cut-off frequency.

Fig. 6 illustrates a practical embodiment of one form of the electrical wave filter of the invention which has been used with satisfactory results. The same reference numerals appearing in Figs. 6 and 1 are intended to designate identical parts.

' In this embodiment, the capacitance between the outer conductor 2 and the surface of zero radio frequency potential 5 has been uniformly distributed over the lengthof conductor 2; by means of suitable insulation 9 of a desired specific inductive capacity. Outer conductor 2 is here shown mounted on a plate ill which is located adjacent the insulating dielectric 9. Surface 5 is shown as a shield surrounding the transmitting equipment H. A novel feature of this arrangement is the capacitor which couples the inner conductor I to the surface 5 of zero radio frequency potential. This capacitonin effect, consists of a metallic plate it, directly connected to the inner conductor I and also directly connected to the terminal I to which the power line is coupled by a single wire (not shown), the plate 1.3, in turn, being .capacitively coupled to the surface 5 by means of a suitable dielectric 143.3116, by means of another dielectric plate IE to the metallic plate l6, said last plate It being ,co nnected directly to the surface 5 through screws l1. These screwsare, of course, insulated from the plate l3 through which they pass, by means of suitable insulating bushings. It should be noted that inner conductor 5 and the metallic lead comprising terminal I extend and are connected to opposite ends of the plate l3. In this Way a non-inductive lay-passing capacitance is obtained, which is very essential to the proper operation of the filter. If we were to connect terminal 1 directly to l, this result would not be obtained. It should be noted, that the length of conductor 1 is measured from the end plane 3 to the point around the bend at which conductor l connects with plate 53. This method of connection of conductor l, and the similar method of connecting terminal 1 to the plate it, eliminates constant with frequency and made equal to the the use of any lead betweensaid conductors and the capacitors l3, l4, l5, I6, with a consequent avoidance of the inductances present in such leads. It should also be observed that there is only a single connection at each end of the filter which need be connected to the associated circuit elements, the other terminal at each end comprising the surface 5 to which the associated circuits may be connected at any point.

Fig. '7 illustrates, graphically, the attenuation versus frequency curve of a particular single section of filter which has been used in practice. In this particular section, the length of the inner conductor l was one-quarter of a wavelength for the frequency of megacycles, and no attempt was made to match the impedance of the filter to the impedance of the external circuits. The external circuits had impedances of about ohms, a value comparable to power cable circuits, while the low frequency characteristic impedance of the single filter section was about 12 ohms. It will be observed from Fig. 7 that the frequencies above 30 megacycles and on both sides of the '75 megacycle frequency, are appreciably attenuated, ranging from a value of about 35 decibels up to 65 decibels.

Although a single filter section has mainly been shown in the drawings, it willbe understood, of course, that either a single section filter or a plurality of such sections in series, may be used depending upon the attenuation characteristic with frequency that it is desired to obtain. Where a polyphase power supply source is employed, it is preferred that one or more filter sections, in accordance with the invention, be used in each power lead. It is to be understood, of course, that although a concentric line filter as shown in the drawings is preferred, that the invention is not limited thereto since any pair of, substantially parallel conductors employed in the manner hereinabove described may be used without departing from the spirit and scope of the appended claims.

By the term odd multiple of one-quarter wavelength, used in the appended claims, it is to be distinctly understood, is meant any odd multiple including one.

What is claimed is:

1. An electrical wave filter comprising a pair of substantially parallel conductors, one of which has an electrical length approximately an odd multiple of one-quarter wavelength at the frequency to be attenuated, the other conductorbeing capacitively coupled to a surface of zero radio frequency potential at at least a plurality of points throughout its length, said conductors being directly connected together at one end, said one conductor being capacitively coupled to said surface at its other end, the lengths and constants of said conductors and the capa-citances of said coupling between said surface of zero radio irequency potential and said conductors being so proportioned with respect to one another that the electrical Wave filter will have a pass band at certain frequencies desired.

2. An electrical wave filter in accordance with claim 1, characterized in this, that said one conductor comprises the inner conductor of a concentric line whose outer conductor is said other conductor of said pair.

3. An electrical wave filter comprising a pair of substantially parallel conductors, said filter being so constructed that one of said conductors has an electrical length approximately equal to an odd multiple of one-quarter wavelength for 75 face at its end remote from the junction of the the frequency at which maximum attenuation occurs, said other conductor being capacitively coupled to a surface of zero radio frequency potential at at least a plurality of points, means for directly connecting said conductors together at one end, and separate means for capacitively coupling said one conductor to said surface at its other end, the lengths and constants of said conductors and the capacitances of said coupling between said surface of zero radio frequency potential and said conductors being so proportioned With respect to one another that the electrical wave filter will have a pass band at certain frequencies desired.

4. A filter in accordance with claim 3, characterized in this that said conductors are devoid of any concentrated inductive reactance.

5. A filter in accordance with claim 3, includin an input terminal at one end of said conductor and an output terminal at the other end of said one conductor.

6. An electrical filter comprising a concentric line having an inner and an outer conductor directly connected together at one end, said inner conductor having an electrical length approximately equal to an odd multiple of one-quarter wavelength for the frequency at which said filter gives maximum attenuation, means for uniformly coupling said outer conductor throughout its length capacitively to a surface of zero radio frequency potential, and means for capacitively coupling the other end of said inner conductor to said surface, the lengths and constants of said conductors and the capacitances of said coupling between said surface of zero radio frequency potential and said conductors being so proportioned with respect to one another that the electrical wave filter will have a pass band at certain frequencies desired.

7. A filter in accordance with claim 6, characterized in this that said last means consists of a concentrated capacitance.

8. An electrical filter comprising a concentric line having an inner and an outer conductor directly connected together at one end, said inner conductor being longer than said outer conductor and having an electrical length approximately equal to an odd multiple of one-quarter wavelength for the frequency at which said filter gives maximum attenuation, means for uniformly coupling said outer conductor throughout its length capacitively to a surface of zero radio frequency potential, and a condenser having one electrode connected to said surface of zero radio frequency potential and another electrode connected to said inner conductor, said quarter wavelength distance of said inner conductor being measured from said last electrode of said condenser to the end of said inner conductor which is directly connected to said outer conductor, the lengths and constants of said conductors and the capacitances of said coupling between said surface of zero radio frequency potential and said conductors being so proportioned with respect to one another that the electrical wave filter will have a pass band at certain frequencies desired.

9. An electrical wave filter comprising a pair two conductors, the lengths and constants of said conductors and the capacitances of said coupling between said surface of zero radio frequency potential and said conductors-being so proportioned with respect to one another that the electrical wave filter will havea pass band at certain frequencies desired.

10. An electrical wave filter comprising an outer conductor and a concentric-inner conductor capacitively coupled together at one end and directly connected together at their other ends, said outer conductor being capacitively coupled to a surface of zero radio frequency potential at one or more points throughout its length, said inner conductor being capacitively coupled to said surface at its end remote from the junction of the two conductors, the lengths and constants of said conductors and the capacitances of said coupling between said surface of zero radio frequency potential and said conductors being proportioned with respect to one another that the electrical wave filter will have a pass band at certain frequencies desired. I

11. An electrical wave filter comprising two pairs of substantially parallel conductors, each pair being connected together at one end, said two pairs being connected directly together also at said ends, one conductor of one pair and the corresponding conductor of the other pair being capacitively coupled to a surface of zero radio frequency potential at one or more points throughout their lengths, said other conductor of each pair and said surface comprising terminals for said filter, the lengths and constants of said conductors and the capacitances of said coupling between said surface of zero radio frequency potential and said conductors being so proportioned with respect to one another that the electrical wave filter will have a pass band at certain frequencies desired.

12. An electrical wave filter in accordance with claim 11, characterized in this that each pair of conductors comprisesinner and outer concentric conductors, said outer conductor being capaci tively coupled to said surface of zero radio frequency potential.

13. An electrical wave filter comprising two pairs of substantially parallel conductors each pair being capacitively coupled together at one end and directly connected together at their other ends, said pairs being connected together at their directly connected ends, one conductor of one pair and the correspondingly located conductor of the other pair being capacitively coupled to a surface of zero radio frequency potential at one or more points throughout their lengths, said other conductor of each pair and said surface comprising terminals for said filter, the lengths and constants of said conductors and the capacitances of said coupling between said surface of zero radio fraquency potential and said conductors being so proportioned with respect. to one another that the electrical wave filter will have a pass band at certain frequencies desired.

14. An electrical wave filter in accordance with claim 13, characterized in this that each pair of conductors comprises inner and outer concentric conductors, said outer conductor being capacitively coupled to said surface of zero radio frequency potential.

15. An electrical wave filter comprising first, second and third pairs of substantially parallel conductors, each pair being directly connected together at one end, said first and second pairs quency potential at their directly connected ends, means for capacitively coupling the directly connected end of said third pair to one conductor of said first pair, and a capacitive coupling between the corresponding conductor of the second pair and one conductor of the third pair at the ends of said second and third pairs remote from the directly connected ends, the lengths and constants of said conductors, and the capacitances of said couplings between saidconductors being so proportioned with respect to one another that the electrical wave filter will have a pass band at certain desired frequencies.

16. An electrical wave filter in accordance with claim 15 characterized in this that said pairs of conductors each comprise an inner and outer concentric conductor, the inner conductor of said third pair being capacitively coupled to the inner conductor of said second pair, and the inner conductor of said first pair being capacitively coupled to the directly connected ends of said third pair. v

17. An electrical wave filter comprising a wave transmission line of recurrent structure, the recurrent sections being composed of an inner and an outer concentric conductor having dimensions proportioned to pass certain frequencies, said inner conductor being the series element of each section and said outer conductor the shunt element thereof, the lengths and constants of said conductors being so proportioned relative to each other that the electrical wave filter will have a pass band at certain desired frequencies.

18. A low frequency band pass filter capable of passing high currents at ordinary power frequencies and providing 'a desired attenuation at ultra high frequencies, comprising a section of concentric transmission line having an inner conductor and an outer conductor, a direct connection between said two conductors at adjacent points in their lengths, said inner conductor being arranged to be the series element of said filter, means for capacitively coupling said outer conductor at one one more points, along its length and said inner conductor at its end removed from said direct connection to a surface of zero radio frequency potential, the lengths and constants of said conductors, and the capacitances of said coupling between said conductors and said surface being so proportioned relative to one another that said wave filter provides little attenuation below the cut-off frequency of said filter and a sharp rise in attenuation for frequencies immediately above said cut-off frequency.

19. An electrical wave filter comprising a coaxial line having a pair of conductors one disposed within the other, said inner conductor having an electrical length approximately equal to an odd multiple of one-quarter Wavelength for the frequency at which said filter gives maximum attenuation, means for uniformly coupling said being connected to a surface of zero radio freouter conductor throughout its length capacitively to a surface at zero radio frequency potential, and means for capacitively coupling one end of said inner conductor to said surface, the lengths and constants of said conductors and the capacitances of said coupling between said surface of zero radio frequency potential and said conductors being so proportioned with respect to one another that the electrical wave filter will have a pass band at certain frequencies desired.

20. An electrical wave filter comprising a coaxial line having a pair of conductors one disposed within the other, said inner conductor having an electrical length approximately equal to an odd multiple of one-quarter wavelength for the frequency at which said filter gives maximum attenuation, means for coupling said outer conductor to a surface of zero radio frequency potential, and means for capacitively coupling both ends of said inner conductor to said surface of zero radio frequency potential, the capacitance of said coupling between said surface of zero radio frequency potential and said inner conductor being such and said conductors being so proportioned with respect to one another that the electrical filter will have a pass band at certain frequencies. 1

21. An electrical wave filter comprising a coaxial line having a pair of conductors one disposed within the other, said inner conductor having an electrical length approximately equal to an odd multiple of one-quarter wavelength for 22. An electrical wave filter comprising a coaxial line having an inner and an outer conductor coupled together at one end, said inner conductor having an electrical length approximately equal to an odd multiple of one-quarter wavelength for the frequency at which said filter gives maximum attenuation, means for coupling said outer conductor to a surface of zero radio frequency potential, and means for capacitively coupling both ends of said inner conductor to said surface of zero radio frequency potential, the

capacitance of said coupling between said surface of zero radio frequency potential and said inner conductor being such and said conductors being so proportioned with respect to one another that the electrical filter will have a pass band at certain frequencies.

BERTRAM TREVOR. 

